A variety of products produced on high speed production lines require the application of coating material to form a protective layer thereon. For example, the production of metal cans involves dispensing a thin film of lacquer or other protective coating onto the can ends or can bodies to protect the contents of the can against metal contaminants. Commercially available lines for the production of metal cans run at speeds on the order of about 400 to 700 cans per minute, and for some applications a coating dispenser such as a spray gun must be turned on and off at the frequency of the cans moving past the spray gun.
Spray guns for coating the ends and/or interior of metal cans are disclosed, for example, in U.S. Pat. Nos. 4,886,013 and 4,430,886 which are owned by the assignee of this invention. Spray guns of this type have proven to be effective in applying the desired protective coating onto the ends and/or interior of metal cans, even at high line speeds, but the valve mechanism associated with such spray guns which starts and stops the flow of coating material to the cans eventually wears out after a large number of cycles. Periodically, the valve tip, valve seat, seals and other elements of the valve mechanism of the spray gun must be replaced because of wear.
Maintenance of the spray guns employed in high speed production lines such as can coating lines has been a problem in the past. The downtime required to repair or replace worn elements of spray guns is costly, particularly considering the high speed of operation of the production lines in which the spray guns are utilized. One solution to this problem has been to employ spray guns which are modular in construction to reduce the time required for the repair or replacement of various components of the coating apparatus, particularly the valve mechanism and associated seat which turns on and off the flow of coating material discharged from the gun.
One problem with spray guns of this type is that such repairs must be effected "off line", i.e., with the spray gun removed from the production line. This requires the coating supply lines, electric lines and/or air lines associated with the gun to be disconnected, as well as the mounting structure which retains the spray gun in position with respect to the object such as metal cans moving therepast. After the spray gun is repaired, it must then be reattached to the mounting structure and to the various supply lines before operation of the can production line can be resumed. These delays are costly and there is a need for reducing the time required for the repair or replacement of various parts of spray guns used in metal can manufacturing lines and other high volume production lines.
Another aspect of the performance of spray guns of the type disclosed in U.S. Pat. Nos. 4,886,013 and 4,430,886 is the speed with which the valve mechanism is closed, particularly after a relatively large number of cycles. In these spray guns, an armature is connected to the valve mechanism which is operative in response to activation of a solenoid to move the valve mechanism to an open position with respect to the discharge outlet of the spray gun. In order to move the valve mechanism to a closed position, the solenoid is de-energized allowing springs acting upon the armature and/or valve mechanism to return the valve mechanism to its original, closed position. One problem with this construction is that the return springs can fail to move the valve to a closed position quickly enough to avoid drooling or leakage of the coating material from the discharge outlet of the spray gun, particularly after a large number of on/off cycles. As a result, the coating material can be deposited onto areas of the cans and/or the production line where it is not desired.